This invention relates to a portable test instrument, and more particularly to a nuclear gauge for measuring the moisture and density properties of a test material.
Portable nuclear gauges are frequently used in the construction industry for measuring the moisture content and density of soils, soil-stone aggregates, cement, and asphalt. One such gauge which has been commercially available for a number of years is the 3400 Series Surface Moisture-Density Gauge manufactured by Troxler Electronic Laboratories of Research Triangle Park, N.C., the assignee of the present invention.
These gauges utilize the principle of Compton scattering of gamma rays for determining the density characteristics of the test material. The moisture content of the test material is determined by using a neutron source and detecting neutrons which are thermalized by the hydrogen in water present in the test material. Portable test instruments employing these principles are disclosed for example in U.S. Pat. Nos. 3,544,793, 4,525,854, 4,542,472, 4,581,599, and 2,781,453.
The aforementioned type of gauge typically employs nuclear sources having an activity level in the millicurie range. For example, the Troxler 3400 Series moisture-density gauge employs a cesium-137 gamma source containing approximately eight millicurie of cesium-137. The neutron source is a mixture of americium oxide and beryllium metal containing about 40 millicurie of americium-241. Although the quantities of radioactive material contained in these gauges are quite small, and an operator may safely use such a gauge daily without receiving any bodily damage due to radiation, nonetheless, precautions and care should be followed in the operating and handling of such gauges.
Some jurisdictions impose severe restrictions on the activity of the nuclear sources which may be used in nuclear test instruments, and consequently, moisture-density gauges of the type described above cannot be used in such jurisdictions. For example, in Japan the maximum limit for radiation sources is below 100 microcuries. Even in jurisdictions where higher limits are permitted, nuclear instruments are often subject to cumbersome regulations and licensing requirements.
In order to overcome the aforementioned disadvantages and limitations, nuclear moisture-density gauges have been proposed which use nuclear sources of relatively low activity, e.g. in the microcurie range. Gauges employing these low activity nuclear sources are subject to fewer and less stringent restrictions and regulations. However, such gauges have been shown to have relatively low levels of accuracy, which seriously limits their usefulness in commercial applicatios. More specifically, because of the low activity nuclear sources, the count rates are significantly lower and as a consequence, extraneous background radiation imparts significantly higher error than if higher activity sources with higher counting rates were employed.
With the foregoing in mind, it is an important object of the present invention to provide a nuclear moisture-density gauge which uses a low activity nuclear source and which provides significantly better accuracy than heretofore provided.